This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 2006-62064, filed on Jul. 3, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to mask read-only memory (ROM), and more particularly to a mask ROM with coding using final vias formed toward the end of the backend processes.
2. Background of the Invention
Semiconductor memory devices may generally be classified into nonvolatile memory devices which do not lose data even when power is cut off, and volatile memory devices which lose data when power is cut off.
Nonvolatile memory devices may be classified into mask read-only memories (ROMs), which cannot be programmed by a user, and programmable ROMs (PROMs), one-time programmable ROMs (OTPROMs), erasable programmable ROMs (EPROMs), and electrically erasable programmable ROMs (EEPROMs), which all may be programmed by a user.
The mask ROM has data stored therein according to coding represented in a mask as generated by a user. Mask ROMs are classified into NAND-type mask ROMs with coding implemented before fabrication of gate electrodes, and NOR-type mask ROMs with coding implemented after fabrication of gate electrodes.
The NOR-type mask ROM has shorter turn-around time (TAT) than the NAND-type mask ROM because coding for the NOR-type mask ROM is performed after fabrication of the gate electrodes. However, the number of backend processes increases as size of memory chips decreases, functions of memory chips diversify, and performance of memory chips improves, thus increasing TAT.
In the case of a conventional mask ROM having transistors, on/off cells are formed by adjusting a threshold voltage of each transistor. Thus, miniaturizing such a memory chip is difficult. In addition, TAT increases for such a memory chip since the coding is performed before the backend processes begin.
In the case of a conventional mask ROM having typical diodes formed in contacts, miniaturizing such a memory chip is difficult since the size of contacts must be reduced. Doping contacts with decreased size becomes more difficult. In addition, decreasing TAT is difficult since contacts are formed relatively early in the back-end processes.
In the case of a conventional mask ROM having a plurality of diodes formed in wells within a substrate, the diodes are completely formed during front-end processes. Thus, TAT increases with an increase in the number of back-end processes.